Propylene polymers have excellent heat and chemical resistance and, in the moulded form, have desirable mechanical properties such as stiffness and impact resistance. Propylene polymers are therefore an attractive option for the production of caps and closures which are used in a range of applications including sealing of food and drinks containers. In many of these applications, the cap or closure is in contact with the food or drink, undergoes sterilisation and is transported and stored prior to use.
An inherent problem associated with polypropylene caps and closures, however, is that interaction with light and oxygen may cause polymer degradation leading to a loss in mechanical performance as well as to the production of potential leachable compounds (i.e. compounds that may migrate out of the polymer into the food or drink). This is a particular issue in the use of polypropylene caps and closures in the food and drinks industry where leachables may effect the taste and odour of the product within the container. Leachables that are aldehydes or ketones are particularly problematic in this regard.
To prevent polymer degradation, it is commonplace to blend polypropylene with one or more stabilisers such as an antioxidant (e.g. an organophosphite or a hindered phenolic compound) and/or an acid scavenger. These compounds are designed to undergo transformation with oxygen and/or light in preference to the polymer and thus prevent its degradation. The stabilisers as well as the products produced during their transformation are, however, volatile organic compounds (VOCs) that may also effect the taste and odour of products in contact with the polymer.
Exactly the same problem is also encountered with other polymer additives that are commonly used to enhance polymer properties. Antistatic agents are often required to modify the precise properties of a propylene polymer and tailor them to the exact end application envisaged. Antistatic agents are, for example, often required to reduce build up of static electricity during storage. Like stabilisers, however, such agents are VOCs, and may produce VOCs, that potentially impact on the taste and odour of products in contact with the polymer.
As ever polymer processability is also vital. A propylene polymer which has good mechanical properties but which cannot be moulded is of little value. This is particularly important in the case of caps and closures which must be produced with fast cycle times and high productivity in order for the process to be economical. As a result polypropylene is often nucleated to improve its crystallisation and therefore processing properties. Nucleating agents are, however, a yet further source of VOCs that may impact upon taste and/or odour of food and drink that contacts the polymer.
The polymer chemist must therefore strive to provide caps and closures having the required mechanical and processing properties, but which are also organoleptically acceptable, i.e. VOCs are minimised.
Another critical feature of caps and closures is that they should have a high closing efficiency. This ensures that the product, e.g. liquid and/or gas, contained by the cap or closure cannot leak out and prevents any contaminants from entering therein. A high closing efficiency is particularly important for caps and closures designed for use on containers for carbonated drinks wherein the internal pressure of CO2 gas may be quite high.